1. Field of the Invention
The present invention relates to a self-excited oscillation type switching power source device and, more particularly, to a self-excited oscillation type switching power source device which is provided with a start-stop circuit for controlling the starting and stopping of the device.
2. Description of the Related Art
Self-excited oscillation type switching power source devices of the related art provided with start-stop circuits for controlling the starting and stopping thereof are described, for example, in Japanese Unexamined Utility Model Application Publication No. 63-100993, Japanese Patent Application No. 2000-295203 filed on Sep. 27, 2000, and Japanese Unexamined Patent Application Publication No. 11-341802. In a ringing choke converter (RCC) described in Japanese Unexamined Utility Model Application Publication No. 63-100993, a starting voltage is applied to a switching element using a starting resistor connected between an input power source and a switching element. Also, in a switching power source device for a half-bridge converter described in Japanese Patent Application No. 2000-295203, a starting voltage is applied to a switching element using a starting resistor connected between an input power source and the control terminal of the switching element. Furthermore, in a switching power source device described in Japanese Unexamined Patent Application Publication No. 11-341802 (FIG. 10), a starting voltage is applied to the control terminal of a switching element using a thyristor.
FIG. 11 shows a circuit diagram of the half-bridge converter described in Japanese Patent Application No. 2000-295203. The outline of the configuration of the converter circuit will be described below. A series circuit including a first switching element Q1 and a second switching element Q2 is connected in parallel to an input power source Vin. One end of a series circuit including a capacitor C, an inductor L, and a primary winding T1 of a transformer T is connected to the node between the first and second switching elements Q1 and Q2, and the other end is connected to an input power source Vin. A rectification-smoothing circuit is connected to a secondary winding of the transformer T. The transformer T contains a first drive winding T3 for developing a voltage that is substantially proportional to a voltage of the primary winding T1, and a second drive winding T4. A first control circuit A1 is connected between the first drive winding T3 and the first switching element Q1. A second control circuit A2 is connected between the second drive winding T4 and the second switching element Q2. The first and second control circuits control the turning on and off of the first and second switching elements so that both of the switching elements are alternately turned on and off, with the time period during which both of the switching elements are off being interposed between the alternate on and off. Thereby, the switching power source device is self-excitedly oscillated.
In the above-descried configuration, the capacitor C is connected in series with each other between the primary winding T1 and the input power source Vin. Accordingly, this circuit functions as a half-bridge converter.
As a starting circuit, a resistor voltage-dividing circuit including a resistor R2 connected between the input power source Vin and the control terminal of the first switching element Q1, and a resistor R7 connected between the control terminal and the source terminal is used. That is, when the voltage from the input power source Vin is increased, and the divided voltage obtained by dividing the input power source voltage using the resistors R2 and R7 exceeds the threshold voltage of the first switching element Q1, the first switching element Q1 is turned on. When the first switching element Q1 is turned on, current flows through the primary winding T1, and voltage is developed in the first drive winding T3 and promotes the turn-on of the first switching element Q1. Thereafter, a transistor Tr1 of the first control circuit is turned on, and the first switching element Q1 is turned off, so that voltage is developed in the second drive winding T4, and then, the second switching element Q2 is turned on. In this way, the first switching element Q1 and the second switching element Q2 are controlled so as to be alternately turned on and off. Thus, the self-excited oscillation is carried out.
FIG. 12 is a circuit diagram of the half-bridge converter provided with a circuit similar to the starting circuit using a thyristor described in Japanese Unexamined Patent Application Publication No. 11-341802 (FIG. 10). The basic configuration of this circuit is similar to that of FIG. 11. The first switching element Q1 and the second switching element Q2 are controlled so as to alternately turn on and off. Thus, the self-excited oscillation is carried out. The difference between the configurations of FIGS. 11 and 12 lies in the starting circuits. In the circuit of FIG. 12, a thyristor SR is connected to the control terminal of the first switching element Q1. The outline of the operation at starting will be described below.
When the voltage of the input power source Vin is increased, a capacitor C1 is charged via a resistor R1. When the charge voltage exceeds the Zener voltage of a Zener diode Dz, the thyristor SR electrically conducts. Thereby, the charge stored in the capacitor C1 flows in the gate of the first switching element Q1, so that the first switching element Q1 is turned on. When the first switching element Q1 is turned on, the charge stored in the capacitor C1 is shunt-discharged via a resistor R4 and the first switching element Q1. Moreover, current flows in the primary winding T1 so that voltage is developed in the first drive winding T3, and the turn-on of the first switching element Q1 is promoted. Thereafter, the first switching element Q1 is turned off, caused by the first control circuit, and then, the second switching element Q2 is turned on. Thus, the first and second switching elements Q1 and Q2 are controlled so as to alternately turn on and off. That is, the self-excited oscillation is carried out.
The above-described configurations of the related art have the following problems caused during starting and stopping of the oscillation.
(1) Self-Excited Oscillation Type RCC Using Starting Resistor
For starting, power is supplied to a load by repeating the starting caused by the starting resistor and the turn-off of the switching element. Accordingly, when the starting time-period from the turn-off of the switching element to the re-starting is long, the power supply per unit time is small. Thus, a predetermined output voltage can not be obtained for a heavy load, and the starting becomes deficient.
Moreover, there are problems with the stopping, in that the oscillation continues until the input voltage becomes low, and therefore, the current peak value of the primary winding becomes high, which causes the transformer to be saturated.
(2) Self-Excited Oscillation Type Half-Bridge Converter Using Starting Resistor
When the voltage from the input power source is slowly increased, the first switching element Q1 gradually reaches the on-state from its active region. In this case, the change ratio of the current flowing in the transformer becomes small, so that no voltage is generated in the transformer. In this case, the self-excited oscillation is not performed. Thus, the starting is deficient.
If the input voltage becomes low, and the on-duty of the first switching element Q1 becomes too large in the stopping operation, the first control circuit for controlling the on-off of the first switching element Q1 may malfunction.
(3) Self-Excited Oscillation Type Half-Bridge Converter Using Thyristor or Diac
No problems are caused in the starting characteristic even if the input voltage is slowly increased. The converter requires thyristors and diacs which are special, expensive elements. The thyristors and the diacs used in the circuit of FIG. 12 need to have a high withstand voltage characteristic, since a high voltage is applied to these elements until they start to conduct.
As seen in FIG. 12, the electric charge stored in the capacitor C1 is shunt-discharged every time the first switching element Q1 is turned on. Thus, the converter has a problem in that the switching loss is increased.
If the input voltage is decreased during the stopping operation so that the on-duty of the first switching element Q1 becomes too large, the control circuit for controlling the on-off of the first switching element Q1 may malfunction.
In order to overcome the problems described above, preferred embodiments of the present invention provide a switching power source device which is provided with a start-stop circuit that both starts and stops oscillation reliably and without errors.
According to a preferred embodiment of the present invention, a self-excited oscillation type switching power source device includes an input power source, a transformer T, a first switching element Q1, a rectification-smoothing circuit, the input power source, the primary winding of the transformer, and the first switching element being connected in series with each other, the transformer T having a first drive winding for developing a voltage which is substantially proportional to a primary winding voltage and causes the first switching element Q1 to turn on, whereby the on-off operation of the first switching element Q1 is controlled, and thereby, the switching power source device is self-excitedly oscillated, and a start-stop circuit including a switch connected between the control terminal and a switching terminal of the first switching element Q1, and a control circuit which detects an input power source voltage across the input power source, compares it to a predetermined voltage, causes the switch to turn off when it detects that the input power source voltage exceeds the predetermined voltage, whereby the first switching element Q1 is turned on, so that the oscillation is started, and causes the switch to turn on when the control circuit detects that the input power source voltage is equal to or less than the predetermined voltage, whereby the first switching element Q1 is turned off, so that the oscillation is stopped.
Accordingly, when the input power source voltage is increased to exceed a threshold, the switch is turned off, and thereby, a starting voltage is steeply applied to the control terminal of the first switching element Q1, due to a starting resistor, so that the first switching element Q1 is turned on. Thereby, even when the input power source voltage is slowly increased, charges in a sufficient quantity to cause the first switching element Q1 to turn on are steeply supplied. Thus, the starting can be securely carried out. For example, even if an AC 100 V commercial power source is used in error for a power source device designed so as to operate with an AC 230 V commercial power source, the power source device can be prevented from operating with the AC 100 V power source device by setting the starting voltage to be higher than AC 100 V. Thus, the device is prevented from malfunctioning with an incorrect power source.
Also, even if the input power source is turned off so that the input power source voltage is decreased, the saturation of the transformer caused by an increase of the primary current, and the malfunction of the device caused by an increase of the on-duty are reliably prevented.
Both of the starting function and the oscillation-stopping function can be performed by the one control circuit. Therefore, it is not necessary to separately provide two circuits. Overall, the number of components can be reduced. Thus, a switching power source device which is highly efficient, has a very small size, and is very lightweight is provided.
Preferably, the device further includes a feedback circuit which is provided between the first drive winding and the control terminal of the start-stop circuit and feeds back an output voltage from the first drive winding to the control terminal to turn off the switch.
The feedback circuit operates so as to positively feed back voltage developed in the first drive winding to the control terminal of the start-stop circuit. Therefore, only the input power source voltage is applied to the control terminal of the start-stop circuit at starting, and after the oscillation is started, the voltage developed in the first drive winding is applied to the control terminal of the start-stop circuit via the feedback circuit, in addition to the input power source voltage. Thereby, the input power source voltage for stopping the oscillation becomes lower than that for starting. Thus, a hysteresis characteristic can be given to these input power source voltages. This hysteresis is effective in preventing the device from malfunctioning caused by chattering of the input voltage. For example, when a commercial power source voltage is rectified and smoothed, and the obtained voltage is used as the input power source voltage, a ripple voltage having a frequency that is substantially equal to two times of the commercial frequency is generated in the input power source voltage. In this case, if there is substantially no difference between the starting and stopping voltages, and the variation of the ripple voltage is large, the device will malfunction when the input power source voltage comes near the ripple voltage. Thus, the oscillation and the stopping are repeated. Thus, according to preferred embodiments of the present invention, malfunctions and problems caused by chattering are prevented, due to the above-described hysteresis characteristic.
Moreover, the above-described hysteresis characteristic causes the oscillation-stopping voltage to be lower than the starting voltage, irrespective of the power supplied to the output. Thus, even if the oscillation is stopped in the interruption of service which instantaneously occurs, and thereafter, the input voltage is increased again, the starting can be carried out. In this case, if the oscillation-stopping voltage is higher than the starting voltage, the starting will be deficient.
Furthermore, referring to the converter having a configuration in which the capacitor is connected in series with the primary winding of a transformer such as a half-bridge converter, when the first switching element Q1 is slowly turned on, starting from the active region, the change ratio of the current flowing in the transformer is small, so that no voltage is developed in the first drive winding. Thus, the self-excited oscillation is not carried out, and the starting becomes deficient. However, since the positive feedback is carried out by the feedback circuit, the first switching element Q1 can be rapidly turned on, and thereby, the deficiency in starting can be eliminated.
In the case of the ringing choke converter in which starting and turning-off of the switching element are repeated to supply power to a load for starting, the converter can be stably started even if the load is heavy, since the turn-on speed at starting is improved so that the starting period is shortened.
Moreover, the on-time period of the first switching element Q1 can be gradually shortened corresponding to the reduction of the input power source voltage at starting by using appropriate elements for the feedback circuit. Thereby, the device can be prevented from malfunctioning caused by increase of the on-duty, and the oscillation can be also maintained when the input voltage becomes low. Moreover, the retention time of the output voltage can be prolonged. Thus, an interruption of service that instantaneously occurs and continues for a long time can be effectively handled.
Preferably, the switch includes a first transistor, the control circuit includes a second transistor connected to the control terminal of the first transistor, the second transistor when the control circuit detects that the input power source voltage exceeds the predetermined voltage, is turned on, and thereby, the first transistor is turned off, whereby the first switching element Q1 is turned on, and the oscillation is started, and when the control circuit detects that the input power source voltage is equal to or less than the predetermined voltage, is turned off, and thereby, the first transistor is turned on, whereby the first switching element Q1 is turned off, and the oscillation is stopped.
Since the switch includes the first transistor, and the control circuit for the switch includes the second transistor, the number of circuit components is reduced. A switching power source device which is inexpensive, is small-sized, and is light in weight is therefore provided.
Preferably, the second transistor compares a voltage divided of the input power source voltage to the threshold voltage between the base and the emitter, and detects whether the input power source voltage exceeds the predetermined voltage or the input power source voltage is equal to or less than the predetermined voltage, based on whether the voltage by resistor-dividing exceeds the threshold voltage or not.
As a way of comparing the input power source voltage to the predetermined voltage, the threshold voltage between the base and the emitter of the second transistor is used. Thus, it is not necessary to use elements such as a comparator, a Zener diode, and so forth. Accordingly, a switching power source device which is inexpensive, is small-sized and is light in weight is realized.
Preferably, the circuit connected to at least one of the base and the emitter of the second transistor has a Zener diode for correcting the temperature characteristic of the voltage between the base and the emitter of the second transistor connected thereto.
In general, the voltage between the base and the emitter of a transistor has a temperature characteristic of about xe2x88x922.0 mV/xc2x0 C. Therefore, when the voltage between the base and the emitter of the transistor is used to detect and compare an input power source voltage, the starting voltage changes with temperature. To correct this, the sum of the voltage between the base and the emitter of the transistor and the Zener voltage of a Zener diode is used as a reference voltage. For example, a Zener diode having a temperature characteristic of about +2.0 mV/xc2x0 C. is used. Thus, the variation of the reference voltage becomes substantially zero. Thereby, the variation of the starting voltage caused by temperature is minimized.
Preferably, the feedback circuit includes a circuit for applying a voltage developed in the first winding to the second transistor, the circuit including at least a resistor and a diode connected in series with each other.
Since the diode is used, no current flows in the feedback circuit at starting. Thus, the starting voltage can be easily determined, not considering current flowing in the feedback circuit. Moreover, the feedback circuit can be simply constructed using a reduced number of parts. Accordingly, a switching power source device which is inexpensive, is small-sized and is light in weight is provided.
Preferably, the feedback circuit includes a circuit for applying a voltage developed in the first winding to the second transistor, the circuit including at least a resistor and a capacitor connected in series with each other.
Since the capacitor is used, DC current at starting does not flow into the feedback circuit. Thus, the starting voltage can be easily set. After the oscillation is started, AC current is caused to flow, and thereby, the feedback can be applied. Similarly to the above-described feedback circuit including the resistor and the diode, this feedback circuit can be simply constructed using a reduced number of parts. Accordingly, a switching power source device which is inexpensive, is small-sized and is light in weight is realized.
Preferably, the feedback circuit includes a circuit for applying a voltage developed in the first winding to the second transistor, the circuit including at least a diode, a resistor, and a Zener diode connected in series with each other, and a capacitor connected in parallel to the diode.
The capacitor connected in parallel to the diode causes the stored charge to be positively fed back at starting, increasing the feedback factor, and can increase the turn-on speed of the first switching element Q1. Moreover, the Zener voltage of the Zener diode connected in series can be optionally set. Thus, the flexibility for setting the oscillation-stopping voltage is greatly improved.
According to another preferred embodiment of the present invention, a self-excited oscillation type switching power source device includes the switching power source device according to preferred embodiments described above in which a second switching element Q2 is connected in series with the first switching element Q1 in such a manner that the series circuit including the first and second switching elements Q1 and Q2 is connected in parallel to the input power source, one end of a series circuit including a capacitor C, an inductor L, and the primary winding of the transformer is connected to the node between the first and second switching elements Q1 and Q2, and the other end of the series circuit is connected to the input power source, a first diode D1 and a first capacitor C1 connected in parallel to the first switching element, a second diode D2 and a second capacitor C2 connected in parallel to the second switching element, a second drive winding which is provided in the transformer T and develops a voltage being substantially proportional to the primary winding voltage to cause the second switching element Q2 to turn on, in addition to the primary drive winding which is provided for the transformer and develops a voltage being substantially proportional to the primary winding voltage to causes the first switching element Q1 to turn on, and a switching control circuit which causes the first and second switching elements to alternately turn on and off with a period of time while both of the switching elements are off being interposed between the alternate on and off, whereby the switching power source device is self-excitedly oscillated.
The switching power source device configured as described above is preferably a converter having a half-bridge configuration. In the converter having the half-bridge configuration, when the input power source voltage is slowly increased, the starting may become deficient. According to this preferred embodiment of the present invention, even if the input power source voltage is slowly increased, the first switching element Q1 is rapidly turned on, so that the change ratio of current flowing in the primary winding of the transformer is increased, and voltage is developed in the primary drive winding. Thus, the self-excited oscillation can be securely started. Moreover, regarding stopping of the device, the input voltage is reduced, and the on-duty of the first switching element Q1 is increased. Thus, before the first control circuit for controlling the on-off of the first switching element Q1 malfunctions, the oscillation is stopped, or the malfunction is prevented by shortening the on-time period. Moreover, the capacitor C1 shown in FIG. 12, which is shunt-discharged every time the switching element is turned on, is not provided. Accordingly, the switching loss can be reduced. A switching power source device which is highly efficient, is small-sized, and is light in weight is realized.
Preferably, the leakage inductor of the transformer is used as the inductor. Since the leakage inductor of the transformer T is used as the inductor L, the number of parts is reduced. Accordingly, a switching power source device which is inexpensive, is small-sized and is light in weight is realized.
Preferably, as the first and second switching elements Q1 and Q2, field-effect transistors are used, and the first and second diodes and the first and second capacitors include the parasitic diodes and the parasitic capacities of the field-effect transistors, respectively.
Since the field-effect transistors are used as the switching elements, the parasitic diodes and the parasitic capacities of the field-effect transistors can be used as the circuit components. Thus, the number of parts is reduced. Accordingly, a switching power source device which is inexpensive, is small-sized and is light in weight is realized.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.